The present dedicated generator for large civil aerospace engines generally takes the form of a “Constant Current” PMA (permanent magnet alternator). The machine is essentially a 3 phase permanent magnet generator having multiple output sets that provide independent power to the EEC channels. On older designs two independent 3 phase outputs were required but on more modern designs 4 independent 3 phase outputs are required.
FIG. 1 shows, in schematic form, the elements of such a generator. The generator 102 is mechanically connected to a gearbox pad 101, and electrically connected via looms 103 to an EEC regulator 104 which supplies the power to the downstream loads.
The “Constant Current” nomenclature is denved from the feet that the short circuit current from the machine is by design set at a value determined by its own self inductance. This value is such that, the machine can safely be run continuously into a short circuit without overheating and the regulation scheme relies on this to rectify and pulse width modulate (PWM) this fixed current to the load in such a way as to maintain the desired output voltage. The current chosen must be adequate to power the load at the lowest operating speed. This configuration is shown in FIG. 2.
This Constant Current PMA has a number of advantages:                1. Safe short circuit current in the event of a fault        2. Simple regulation algorithm        3. Power can be made available over a wide speed range (6%-120%)        4. Semiconductor Switches are run over a restricted range of current and voltage        5. System is self starting, no external power required        
However, the Constant Currant PMA also suffers from a number of disadvantages                1. The losses in the generator are fixed independent of the actual load.        2. The maximum power must be fixed during system design; it is not possible to transiently increase the output power. This means that any margin, however transient has to be designed in from day one increasing risk, size and weight.        3. The machine is never more than 50% efficient.        4. The machine has an undesirable high open circuit failure mode.        5. These machines are reaching the limit of manufacturability.        
Therefore, whilst the present dedicated generator scheme, based on a PMA, is robust and well proven, the need for increased power to the EEC, driven by more complex engine control schemes, that in themselves are driven by the desire for improved engine efficiency, mean that the size and weight of the PMA has been rising whilst the reliability has been falling due to reducing thermal margins driven by gearbox temperature and internal PMA losses.
Although efforts are being made to investigate improvements to the PMA to overcome some of the manufacturing issues and to mitigate thermal issues, the basic power limitation issues will remain.
An object of the present invention is to provide an alternative generator that may reduce the size and weight of the dedicated power source whilst raising the efficiency such that temperature margins are restored and so improving reliability.
One such generator is the Switched Reluctance Generator (SRG) which is derived from the Switched Reluctance Motor (SRM). These machine types are well reported in the literature and have been trialed for a number of engine based applications due to a range of inherent advantages. However, they also present a number of technical challenges.
In particular, the control of such generators is typically highly complex and reliant on a shaft encoder and microprocessor or Digital Signal Processor (DSP) to provide timing data to control the current flows in the machine windings. This type of control is not acceptable in a dedicated power application where the reliability of the overall system must be higher than that which can be achieved if the system is reliant on a resolver and complex logic for its operation.
A switched reluctance (SR) motor or generator consists of a number of windings designed to create flux paths within the machine that change in length, or reluctance, as the machine moves or rotates.
The magnetic concept of “reluctance” represents the difficulty that flux has in completing a complete circuit. Most energy in the machine is stored in these reluctance elements and it is the change in reluctance (effectively length) of these flux paths that drives the energy conversion process from mechanical to electrical within a SR machine. It may be noted the flux paths within the machine are almost completely independent of each other and this gives a suitably controlled SR machine inherent fault tolerance.
For the purposes of this description it will be assumed that the energized windings are on the outside of the machine and that this is the “stator” whilst the inner element is supported by bearings and is the “rotor”. It will also be assumed that the flux will pass radially across the machine. All the above are in practice variables within the machine design which are readily changed by the skilled person and the present invention is not intended to be limited to this configuration.
FIG. 3 shows a typical arrangement for a simple Switched Reluctance (SR) machine. It shows the two inner magnetic paths of the rotor 105 which, when in alignment, complete the flux path for the stater coils 106. In the example shown in FIG. 3, there are three “coil pairs” denoted by the numbers 1-6. Each pair of coils is positioned opposite each other in the stator 107.
As a motor the SRM can be understood quite simply as a machine driven by magnetic attraction, so a current flow is a created in one of the coil pairs that creates a magnetic field that attracts the nearest rotor element until it pulls it into line. Once this pole pair is aligned the next pair of coils in the required direction of rotation is energized and the rotor moves to the next position and so the process proceeds. For example, from the position shown in FIG. 3 energizing coils 1 and 4 would attract the rotor poles and drive the rotor counter-clockwise whilst energizing coils 2 and 5 would attract the rotor in a clockwise direction.
In practice the timing and current control of the coil energization is critical to achieving the best performance from an SRM and controller boards based on DSPs and accurate position sensors attached to the rotor shaft are therefore used to optimize the drive function. Much work has been done, again using DSP based solutions, to achieve sensorless control of SRMs but difficulties in operating over the full speed range still limit their usefulness except for fall back control needed if the sensors fail.
FIG. 4 shows the electrical equivalent of the SRM in FIG. 3 for counter-clockwise rotation. As can be seen, each pair of coils 106 are connected in series between the positive and negative supply tines. A pair of switches 103 are used to energise each pair of coils in accordance with the switching phase sequence shown at the top of the Figure. A current path when the switches are open and the phase is in freewheeling or generating mode is provided by diodes 108.
SR machines are noted for their ruggedness and ease of construction, both ideal features in an aerospace application. However they are often not selected for these applications due to three key design issues.
First SR machines are inherently less efficient than a permanent magnet machine. However, as discussed above, in the aerospace application, the permanent magnet solution can never be more than 50% efficient because of the nature of the impedance limited power transfer they employ so an SR machine operating at >85% efficiency can easily out compete the PMA on this key weakness.
Second the SR family of machines is critically dependent on the bearing type used and the concentricity of the bearing to the stator and rotor pole tips. In embodiments of the present invention we propose, there is a significant efficiency margin, and so slightly increased air gaps at the pole tips can be incorporated to overcome this problem by maintaining proportionately better balance of forces as the pole tips come into alignment.
The final problem often cited against the SR generator is the need for complex control and sensor based solutions that inherently reduce the system reliability below acceptable levels for a dedicated power source. The generators and methods of operation of the present invention aim to address this problem.
It is an objective of this invention to provide a generator and a method of controlling a generator which avoid the need to have, whether by absolute measurement, or computed by indirect means, the relative position of the rotor to the stator of the SRG. It is a further object of the present invention to use the actual instantaneous generation of power from the machine to effect control.